Rochester B is a manual
choke and will
generally be found on pickups with a 6 cyl. engine.
BC is an automatic choke, also
referred to as a hot air choke. It is a hot air choke because hot air
from the exhaust manifold is brought up through a hot air tube to the
choke thermostat. The movement of this hot air is facilitated by a
vacuum draw from the carburetor. This design is usually found on
passenger cars w/ a 6 cyl engine.
BV type of carburetor includes a
vacum break, or choke pull off. This is used to pull the choke off when
under load and the engine is still cold.
BC & BV carburetors are identified by a tag attached to the top
of the carburetor. Look for a number that starts with 7. Example 702140.
Rochester B, BC, BV carburetors had the last
part of the carburetor number stamped on the bottom of the mounting
flange. There will be either 4, or 5 numbers. In this case
there are 5 numbers, 23105. Put 70 in front of the number, 703105. Had
there only been only 4 numbers, then 700 would be added to the
number as in: 7002301
All other numbers stamped on
the carburetor are
nothing more than casting numbers for the part it is stamped on and in
most cases isn't enough to determine which parts to use.